The present invention relates generally to wavelength division multiplexed optical signals, and more particularly, to minimizing the thermal and pressure effects on diffraction grating based spectrometer systems incorporating wavelength division demultiplexing devices.
The telecommunications industry has grown significantly in recent years due to developments in technology, including the Internet, e-mail, cellular telephones, and fax machines. These technologies have become affordable to the average consumer such that the volume of traffic on telecommunications networks has grown significantly. Furthermore, as the Internet has evolved, more sophisticated applications have increased data volume being communicated across telecommunications networks.
To accommodate the increased data volume, the telecommunications network infrastructure has been evolving to increase the bandwidth of the telecommunications network. Fiber optic networks that carry wavelength division multiplexed optical signals or channels provide for significantly increased data channels for the high volume of traffic. The wavelength division multiplexed optical channels or polychromatic optical signals comprises monochromatic optical signals. The wavelength division multiplexed optical channels carry time division multiplexed data containing information, including voice and data. Contemporary optical networks can include forty or more monochromatic optical channels on a single fiber and each monochromatic optical channel can carry many thousands of simultaneous telephone conversations or data transmissions, for example.
An important component of the fiber optic networks is an optical performance monitor (OPM) for monitoring the performance of the optical system. The OPM provides a system operator the ability to monitor the performance of the individual substantially monochromatic optical signals. The optical performance monitor may measure the following metrics: power level, center wavelength, optical signal-to-noise ration (OSNR), interference between channels such as crosstalk, and laser drift. By monitoring these metrics, the optical network operator can identify and correct problems in the optical network.
The OPM may include a dispersion engine and an optical sensor. The dispersion engine may include lenses and a dispersion device, such as a diffraction grating. The lenses process the polychromatic optical signal and cause the polychromatic optical signal to be incident to the dispersion device at a near-Littrow condition, which is a condition where the angle of the incident light beam is reflected back toward the source of the incident light beam near the incident angle at at least one wavelength. The dispersion device diffracts the polychromatic optical signal into its component substantially monochromatic optical signals, which are diffracted at angles as a function of the wavelength of each substantially monochromatic optical signal. Each substantially monochromatic optical signal forms a spot that is focused at distinct locations along the optical sensor.
Both the mechanical and optical components of the spectrometer are affected by changes in temperature. They expand and contract changing in relative position, and also changing in optical properties. Additionally, changes in pressure cause changes in optical properties of air within the spectrometer. These changes must be calibrated out or they will affect the quality of the information received from the spectrometer. Thus, it is desirable to minimize the effects of temperature and pressure on the spectrometer.
To overcome the adverse affects of changes in temperature and pressure a device for monitoring wavelength divisions multiplexed optical signals has been athermalized and desensitized to pressure. The device can also be part of an optical network. The device has a structure for supporting components of the device. An optical component is supported at one end of the structure for transmitting the optical signals. A diffraction grating is supported at an opposing end of the structure for diffracting the optical signals from the optical component. An optical sensor is supported in relation to the diffraction grating by the structure for monitoring the optical signals. A lens assembly is supported by the structure and disposed between the optical sensor and the diffraction grating. The lens assembly has a focal length for focusing the optical signals in relation to the optical sensor. The diffraction grating has an angular dispersion that changes with temperature and the focal length changes with temperature. The product of the focal length and angular dispersion remains substantially constant with temperature. Optionally, this can be calibrated with software and a temperature sensing system.
The spectrometer further includes a prism supported by the structure and disposed between the lens assembly and diffraction grating. The prism has an angular dispersion that changes with temperature. The product of the focal length and the sum of the angular dispersion of the prism and the angular dispersion of the grating remains substantially constant with temperature. The change in index of refraction with temperature of the prism is a value approximately equal to the negative value of the coefficient of thermal expansion of the diffraction grating. A change in index of refraction with temperature of the prism is substantially within 30% of a negative value of a coefficient of thermal expansion of the diffraction grating. The prism is configured to anamorphically compress the diffracted optical signals. A first prismatic region formed between the prism and the lens assembly is opposed to a second prismatic region formed between the prism and the diffraction grating. The first prismatic region has a first angle measured between the lens assembly and the prism and the second prismatic region has a second angle measured between the prism and the diffraction grating, the second angle being approximately equal to the first angle.
In an embodiment without a prism, the coefficient of thermal expansion of the diffraction grating is a value chosen to be approximately equal to a negative of the change in index of refraction with temperature of air. In this case, the diffraction grating has a coefficient of thermal expansion of approximately 0.5 PPM/degree Celsius to 1.5 PPM/degree Celsius.
The lens assembly is constructed of a material chosen to minimize its variance in focal length over temperature. The assembly comprises a telephoto lens. A coefficient of thermal expansion of the structure and a change in index of refraction with temperature of the lens assembly are values selected so that a length of the structure changes substantially proportionally with the focal length of the lens assembly in response to temperature changes, whereby the lens assembly remains substantially focused in relation to the optical sensor.